Adaptive gain adjustment for electromagnetic devices

ABSTRACT

A control system for the adjustment and calibration of electromagnetic devices, such as E-I core electromagnetic devices, using adaptive gain adjustment. A controller is provided with an input current and an output force and provides an output signal indicative of a force gain estimate, wherein the force gain estimate is the ratio of the output force to the input current.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates generally to control systems, andmore particularly, the present invention relates to adjustment andcalibration of electromagnetic devices.

[0003] 2. Description of Related Art

[0004] Electromagnetic devices are well known. One example is an E-Icore actuator, which is a type of electromagnetic linear motor so namedbecause of its two main components. The first component is the E-core,which is a three-barrel structure having a shape that resembles theletter “E” with an insulated electric coil wire wound around the centerbar and a source of current supplying current to the coil. Currentrunning through the coil creates an electromagnetic field which attractsan associated I shaped core. Thus, an electromagnetic force is exertedacross the width of a gap between the E-core and the I-core. When aconstant current is supplied to the coil, the force of theelectromagnetic field may change as the gap distance changes. Thischange in force is often referred to as the output force gain of the E-Icore system.

[0005] E-I core electromagnetic devices may be used to precisely adjustthe position of an object. Unlike, for instance, a bi-directional voicecoil motor which also provides precision positioning, E-I coreelectromagnetic devices use substantially less electric current andtherefore less energy in the form of waste heat. Another benefit of E-Icore electromagnetic devices is the reduction of vibration duringprecision motion. For instance, precision motion is frequently needed inmachining, lithography, and other strict tolerance manufacturingapplications e.g., in stepper and scanner machines used in thesemiconductor industry. Typically, the goal is to provide preciseadjustment of, for instance, a sampler or work piece stage in threedimensions.

[0006] In the prior art, calibration and adjustment is often donethrough a mechanical adjustment, which has been found to be timeconsuming and imprecise, especially due to problems of driftattributable to thermal or other effects. This both degrades performanceand reduces system throughput, since time is required for the actualcalibration. An improved calibration method would be very desirable forsuch systems.

[0007] When using an E-I core electromagnetic device, the output forcegain may be used to calibrate the device for precision adjustments, suchas for positioning components of a precision machine. Preciselypositioning machine components is difficult because the output forcegain varies due to effects such as part-to-part variance, geometricmounting inaccuracy, and a dynamically changing gap distance duringoperation of E-I core electromagnetic device. Using a gap distancemeasurement, E-I core commutation equations may be used to model theoutput force gain. These models require burdensome hand tuning for eachE-I core electromagnetic device. Further, even with a model, it isdifficult to obtain precise measurements when dealing with large rangesof gap distance. Moreover, equations used in models, such as force gainmodel equations, become ineffective when gap distance information isunavailable or the force gain changes due to unmodeled factors.

[0008] Thus, there is a need for an improved method of modeling outputforce gain in electromagnetic devices to create precise measurements andadjustments. Further, there is a need for an improved method ofcalibrating the output force gain for E-I core electromagnetic devices.

SUMMARY OF THE INVENTION

[0009] Systems and methods consistent with embodiments of the presentinvention provide for a controller for an electromagnetic device. Inaccordance with one embodiment of the invention, a controller isprovided with a first input port adapted so as to receive dataindicative of an input current; a second input port adapted so as toreceive data indicative of an output force; and an output port adaptedso as to provide an output signal indicative of a force gain estimate,wherein the force gain estimate is the ratio of the output force to theinput current.

[0010] In accordance with another embodiment of the invention, acontroller electrically coupled to an E-I core electromagnetic device isprovided with a first input port adapted so as to receive dataindicative of an input current; a second input port adapted so as toreceive data indicative of an output force; a processor for generating aforce gain estimate, wherein the force gain estimate is the ratio of theoutput force to the input current; and an output port adapted so as toprovide an output signal indicative of an adjusted current, wherein thecurrent is adjusted based on the ratio of a constant force gain and theforce gain estimate.

[0011] In accordance with another embodiment of the invention, an E-Icore electromagnetic device comprising, a controller electricallycoupled to the E-I core electromagnetic device to control the electriccurrent to the device, wherein the controller generates an adjustmentvalue from the ratio of a constant force gain to a force gain estimate,wherein the force gain estimate is ratio of an output force and an inputcurrent.

[0012] In accordance with another embodiment of the invention, anapparatus is provided with a first assembly including an E-core of anelectromagnetic device; a second assembly including an I-core of theelectromagnetic device which cooperates with the E-core and is locatedadjacent to the E-core; a third assembly including a force sensorattached to E-core to supply the output force signal; and a controllercoupled to the electromagnetic device to control a current to theE-core, wherein the controller controls the current based on the ratioof a constant force gain to the force gain estimate.

[0013] It is to be understood that both the foregoing generaldescription and the following detailed description are exemplary andexplanatory only and are not restrictive of the embodiments of thepresent invention, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The accompanying drawings are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention. In the drawings,

[0015]FIG. 1 is an illustration of an E-I Core electromagnetic devicemounted with a force sensor consistent with an embodiment of theinvention;

[0016]FIG. 2 is a block diagram of the real-time E-I core force gainestimate scheme consistent with an embodiment of the invention;

[0017]FIG. 3 is an illustration of a CMP force-control prototypeconsistent with an embodiment of the invention;

[0018]FIG. 4 is a time history graph of gap distance and force gainestimate, illustrating a problem to be solved;

[0019]FIG. 5 is a block diagram of a CMP force-control loop with anadaptive gain adjustment servomechanism consistent with an embodiment ofthe invention;

[0020]FIG. 6 is a time history graph of gain adjustment value and gapdistance consistent with an embodiment of the invention;

[0021]FIG. 7 is a graph of open-loop frequency response at three gapdistances without adaptive gain adjustment (“AGA”) consistent with anembodiment of the invention;

[0022]FIG. 8 is a graph of closed-loop frequency response at three gapdistances without AGA consistent with an embodiment of the invention;

[0023]FIG. 9 is a graph of open-loop frequency response at three gapdistances with AGA consistent with an embodiment of the invention; and

[0024]FIG. 10 is a graph of closed-loop frequency response at three gapdistances with AGA consistent with an embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] Reference will now be made in detail to the presently preferredembodiments of the present invention, examples of which are illustratedin the accompanying drawings.

[0026] In one embodiment of the invention, adaptive gain adjustment(“AGA”) for an E-I core electromagnetic device uses a force gainestimate to adjust controller output in real time. The AGA is a step inthe control system supplying calibration and adjustment ability to thesystem without hand tuning. AGA corrects for changes in gap distancemeasurements, making complicated models unnecessary, and providing E-Icore electromagnetic devices with precision control under differentconditions, such as gap distance, tilt angle, or part-to-part variance.AGA also provides a tool to calibrate the output force gain of an E-Icore electromagnetic device.

[0027] Embodiments of the present invention may be implemented inconnection with various types of E-I core electromagnetic devices used,for example, in precision force and motion control applications. The E-Icore electromagnetic device is operated by exerting an electromagneticforce across a gap, and may be a non-contact force or position controldevice. By way of a non-limiting example, an exemplary implementationwill be described with reference to Chemical Mechanical Polish (CMP). Ascan be appreciated by those skilled in the art, embodiments of theinvention can be implemented for other types of E-I core electromagneticdevices, such as for actuators in scanning lithography systems.

[0028]FIG. 1 is an illustration of an exemplary E-I core electromagneticdevice that provides non-contact force or position control. The maincomponents of an exemplary E-I core electromagnetic device setup includea fixed E-core 110, a coil 120, a movable I-core 130, and a force sensor140.

[0029] Fixed E-core 110 may be any type of core material for use with acoil. In one embodiment, E-core 110 may be a C-core. In anotherembodiment, E-core 110 may be a 5-pronged core. Coil 120 may be any coilthat creates a circulating magnetic field. I-core 130 may be any type ofmetal or other material capable of responding to a force filed generatedby Coil 120. In one embodiment, I-core 130 may be connected to astructure.

[0030] Force sensor 140 may be used for control purpose. Examples offorce sensor 140 include standard contact sensors, such as a load cellor a strain gauge, which are standard force sensors known to those inthe art.

[0031] Each E-I core electromagnetic device has a force constant (c)associated with the E-I core actuator design. An input current (I) runsthrough coil 120. The current creates an output force (F) between fixedE-core 110 and moveable I-core 130. In FIG. 1 the gap distance is apreviously measured value, x. In some embodiments the gap distancemeasurement is available from a sensor. f(x) is the gap function, whichmodels the geometry of the E-I core design and setup. For an ideal flatsurface E-core and I-core in parallel, the gap function can be as simpleas f(x)=x². Other more complicated gap functions may be expressed fordifferent designs and mounting geometries.

[0032] With the input current, I, force constant, c, and gap distance,x, between E-core and I-core, the output force F is usually estimated bythe commutation equation: $\begin{matrix}{F = {c \cdot \frac{I^{2}}{f(x)}}} & (1)\end{matrix}$

[0033] In practice, however, the commutation equation is difficult toestimate for ranges of gap distance due to movement of the I-core.Further, a commutation equation model needs tuning and verification.

[0034] The output force gain is a function of the gap distance, and maybe represented by the function G(x). At a discrete time-step, k, the gapdistance is represented by the function x(k). The output force gain isthen G(x(k)), and can be represented by: $\begin{matrix}{{G\left( {x(k)} \right)} = \frac{c}{f\left( {x(k)} \right)}} & (2)\end{matrix}$

[0035] At the k-th time-step, the force output, F(k), with an inputcurrent, I(k), can be rewritten in discrete-time domain as:

F(k)=G(x(k))·I ²(k)  (3)

[0036]FIG. 2 illustrates how the output force gain, G(x(k)), can be usedto determine a force gain estimate, Ĝ(k). The force gain estimate usesthe input current, I(k), to generate the input command, u(k), which canbe calculated by:

u(k)=I ²(k)  (2)

[0037] Given the input current, I(k), and an output force, F(k), theforce gain, G(x(k)), can be computed directly by: $\begin{matrix}{{G\left( {x(k)} \right)} = \frac{F(k)}{u(k)}} & (3)\end{matrix}$

[0038] Because the digital signal I/O loop inherits one time-step delay,the force gain estimate, Ĝ(k), at the k-th time-step can be obtained by:$\begin{matrix}{{\hat{G}(k)} = {\left( \frac{F(k)}{u\left( {k - 1} \right)} \right)*{h(k)}}} & (4)\end{matrix}$

[0039] where asterisk mark (*) means discrete-time convolution sum, andh(k) represents a low-pass filter added in the signal path to reduce thesignal noise.

[0040] The force gain estimate relieves the commutation equations fromthe need for gap distance measurements. This force gain estimate canthen be incorporated as an adjustment value into AGA control step or canstand alone as a tool for calibrating for each E-I core electromagneticdevice. It is to be understood that this process is typically carriedout by a microprocessor (or microcontroller), which is properlyprogrammed and typically resident in such a system for purposes ofcontrolling E-I electromagnetic devices. This microprocessor may be partof a conventional feedback loop controlling the device. Of course, onedoes not require a microprocessor or microcontroller to carry out thefunctions, but this process may be performed, for instance, byhard-wired circuitry or other control circuitry instead. A computerrather than a microprocessor or microcontroller could also perform thefunctions.

[0041] Embodiments of the present invention may be implemented inconnection with various types of E-I core electromagnetic devices invarious applications. By way of a non-limiting example, an exemplaryimplementation will be described with reference to a Chemical MechanicalPolish (“CMP”) application. As can be appreciated by those skilled inthe art, embodiments of the invention can be implemented using othertypes of electromagnetic devices or in other applications where anon-contact force or position control device is appropriate, such asactuators in scanning lithography systems.

[0042]FIG. 3. illustrates a CMP force-control prototype. The prototypeincludes a base 310, a circular I-core ring 320, and E-core 330. CMP isapplied to base 310 with a passive force load 350 using circular I-corering 320. In one embodiment, passive force load 350 may be a passiveinaccurate air-pressure force load. During operation, circular I-corering 320 is attached with a polish pad 370, which rotates above base310. E-core 330 is mounted above circular I-core ring. This E-I coredevice is used to control the passive force load 350 applied to rotatingI-core ring 320. Due to the imperfect surface and the vibration ofI-core ring 320, rotating I-core ring changes the gap distance 340dynamically, directly affecting the output force gain. E-core 330 can beused to precisely control the polishing force on rotating I-core ring320, to insure a smooth and even polish. Varying the current provided tothe coil of E-core 330, creating a counter force 360 may preciselycontrol the applied polishing force. The adaptive gain adjustmentadjusts the current in order to insure a consistent applied force, evenwith variations in gap distance.

[0043]FIG. 4 is a time history plot of gap distance and force gainestimate during operation of the E-I core in the CMP prototype. Thisgraph illustrates the problem of a widely varying force generated duringthe operation of an E-I core electromagnetic device. That kind ofvariation is a problem in designs needing uniform performance. In thisembodiment, the rotating speed of I-core ring 320 is 140 RPM, andconstant 2-amp current is supplied to E-core 330. The graph tracks theforce gain estimate, as the gap distance varies in between 720 μm and930 μm. The force gain estimate changes dynamically in between 830 μm to1200 μm. This type of gain variation affects output performance, and intraditional systems is a challenge to the designs for uniform andoptimal performance.

[0044]FIG. 5 illustrates how AGA may be used in a CMP force controlloop. This embodiment includes utilizing a real-time force gain estimate520 and an adaptive gain adjustment block 530 in the force control loop.The E-I core electromagnetic device is calibrating at the preset gapdistance by measuring the constant force gain, G₀. Based on the constantforce gain, G₀, a feedback controller, K(z), is then designed andoptimized. The feedback controller is electrically coupled to the E-Icore. The coupling may be direct, or there may be various componentsbetween the E-I core and the controller. During operation, due to theeffects of output force, F(k), and gap disturbance, w(k), the gapdistance, x(k), will change dynamically, causing the generation of anoutput force gain, G(x(k)). Without adjustment, the force gain variancewill degrade system output performance.

[0045] The experimental setup for the constant force gain, G₀, isobtained at a 800 μm gap distance. When the gap distance is larger than800 μm, the actual force gain, G(x(k)), can be expected lower than G₀,and vice versa.

[0046] In one embodiment, an adaptive gain adjustment block 530 isapplied to the feedback control system to trace and adjust thecontroller gain in real-time to compensate for force gain variance. Thereal-time force gain estimate path 540 generates the force gainestimate, Ĝ(k), to approximate actual force gain, G(x(k)), and sends theforce gain estimate out to the adaptive gain adjustment control block.

[0047] At the adaptive gain adjustment block, the controller gain isestimated as the ratio between the constant force gain, G₀, and theforce gain estimate, Ĝ(k). In one embodiment, the gain adjustment valuemay be bounded in a reasonable range for system stability. The forcegain estimate, Ĝ(k), will tend to cancel out the actual force gain,G(x(k)), to let the system behave as time-invariant with the constantforce gain of G₀, thus preserving output performance during operation.

[0048] In one embodiment, an adaptive gain adjustment block 530 may bepart of the controller. Adaptive gain adjustment block 530 may beimplemented in circuitry, in firmware, or in a microprocessor (ormicrocontroller) that is appropriately programmed.

[0049]FIG. 6 illustrates graphs of experimental results showing the timehistory of gain adjustment and gap distance during operation. The gainadjustment value is the ratio of the constant force gain value to theforce gain estimate. The graph shows how the gain adjustment valuefluctuates in between 0.8 mm and 1.2 mm consistent with the gap distancechange.

[0050] In another embodiment, the force gain estimate may be used forautomatic calibration. Traditionally, a gap distance measurement isusually needed to determine the constant force gain value. Here, aconstant force gain value may be estimated using the force gainestimate. For a system in which gap distance is not known, constantforce gain can be calculated by running the control loop once. In onerun through, the control loop, the force gain estimate can bedetermined. This force gain estimate value is an estimate of theconstant force gain value.

[0051] The following example will demonstrate results from the use ofthe AGA servomechanism. FIG. 7 is an open-loop response of the CMPforce-control system having the same controller K(z) design with threedifferent preset gap distances, 0.5 mm, 0.9 mm, and 1.3 mm. This graphshows open-loop gain shifting attributable to the force gain change.FIG. 8 is the corresponding closed-loop response. It also shows that thebandwidth and performance vary significantly depending on the gapdistance change. In this example, individual controller tuning isnecessary. With the introduction of AGA, FIG. 9 shows the open-loopresponse with no gain-shifting phenomenon at three different gapdistances. The corresponding closed-loop response shown in FIG. 10 alsoindicates the uniform bandwidth and performance regardless of thevarying gap distances, without individual controller tuning.

[0052] While embodiments or features of the invention have beendescribed as a functional block, one skilled in the art will appreciatethat these aspects can also be implemented in the controller or throughinstructions stored in memory or stored on or read from other types ofcomputer-readable media, or in circuitry.

[0053] Furthermore, the above-noted features and embodiments of thepresent invention may be implemented in various environments. Suchenvironments and related applications may be specially constructed forperforming the various processes and operations of embodiments of theinvention or they may include a general purpose platforms selectivelyactivated or reconfigured to provide the necessary functionality. Theexemplary processes disclosed herein are not inherently related to anyparticular computer or other apparatus, and aspects of these processesmay be implemented by a suitable combination of parts. For example,various general purpose machines may be used with programs written inaccordance with teachings of the invention, or it may be more convenientto construct a specialized apparatus or system to perform the requiredmethods and techniques.

[0054] Other embodiments of the invention will be apparent to thoseskilled in the art from consideration of the specification and practiceof the exemplary embodiments disclosed herein. Therefore, it is intendedthat the specification and examples be considered as exemplary only,with a true scope and spirit of the invention being indicated by thescope of the following claims and their equivalents.

What is claimed is:
 1. A controller for an electromagnetic devicecomprising: a first input port adapted so as to receive data indicativeof an input current to an electromagnetic device; a second input portadapted so as to receive data indicative of an output force of theelectromagnetic device; and an output port adapted so as to provide anoutput signal indicative of a force gain estimate, wherein the forcegain estimate is the ratio of the output force to the input current. 2.The controller of claim 1, wherein the electromagnetic device is an E-Icore electromagnetic device.
 3. The controller of claim 1, furthercomprising a low pass filter to process the force gain estimate.
 4. Thecontroller of claim 1, wherein the force gain estimate is a calibrationestimate of constant force gain for the electromagnetic device.
 5. Acontroller electrically coupled to an E-I core electromagnetic device,comprising: a first input port adapted so as to receive data indicativeof an input current to an electromagnetic device; a second input portadapted so as to receive data indicative of an output force of anelectromagnetic device; a processor for generating a force gainestimate, wherein the force gain estimate is the ratio of the outputforce to the input current; and an output port adapted so as to providean output signal indicative of an adjusted current, wherein the currentis adjusted based on the ratio of a constant force gain and the forcegain estimate.
 6. The controller of claim 5, further comprising a lowpass filter to process the force gain estimate.
 7. An E-I coreelectromagnetic system comprising: a controller electrically coupled tothe E-I core electromagnetic device to control the electric current tothe device, wherein the controller generates an adjustment value fromthe ratio of a constant force gain to a force gain estimate, wherein theforce gain estimate is a ratio of an output force and an input current.8. The system of claim 7, wherein the force gain estimate is furtherprocessed with a low pass filter.
 9. An apparatus comprising: a firstassembly including an E-core of an electromagnetic device; a secondassembly including an I-core of the electromagnetic device thatcooperates with the E-core and is located adjacent to the E-core; athird assembly including a force sensor attached to at least one of theE-core or I-core to supply the output force signal; and a controllercoupled to the electromagnetic device to control a current to theE-core, wherein the controller controls the current based on the ratioof a constant force gain to the force gain estimate.
 10. The apparatusof claim 9, wherein the force gain estimate is based on the ratio of aninput current and a force output.
 11. A method for adjusting forirregularities in output force from E-I core electromagnetic devicescomprising: calculating a force gain estimate, wherein the force gainestimate is the ratio of an output force and an input current;processing the force gain estimate using a low pass filter; andmodifying the current provided to an E-I core electromagnetic devicebased on the processed force gain estimate.
 12. A method for adaptivegain adjustment comprising: generating an adjustment value through theratio of a force gain to a force gain estimate, wherein the force gainestimate is ratio of an output force and an input current; and applyingan adjustment value to the current.
 13. A method for dynamicallyadjusting output force of an E-I core electromagnetic device comprising:providing a current to the E-I core electromagnetic device, generatingan output force, wherein the output force is a function of an amount ofcurrent sourced to the E-I core electromagnetic device; and adjustingthe amount of the current based on a force gain estimate.
 14. The methodof claim 13, wherein the force gain estimate is the ratio of an outputforce and an input current.
 15. A controller for an electromagneticdevice comprising: means for receiving data indicative of an inputcurrent to the electromagnetic device; means for receiving dataindicative of an output force of the electromagnetic device; and meansfor providing an output signal indicative of a force gain estimate,wherein the force gain estimate is the ratio of the output force to theinput current.
 16. The controller of claim 15, wherein theelectromagnetic device is an E-I core electromagnetic device.
 17. Thecontroller of claim 15, further comprising a low pass filter to processthe force gain estimate.
 18. The controller of claim 15, wherein theforce gain estimate is a calibration estimate of constant force gain forthe E-I core electromagnetic device.
 19. A system comprising: anelectromagnetic device; and a controller having: means for receivingdata indicative of an input current to the electromagnetic device; meansfor receiving data indicative of an output force of the electromagneticdevice; and means for generating a force gain estimate, wherein theforce gain estimate is the ratio of the output force to the inputcurrent; and means for providing an output signal for adjusting thecurrent to the electromagnetic device, wherein the current is adjustedbased on the ratio of a constant force gain and the force gain estimate.20. The system of claim 19, further comprising a low pass filter toprocess the force gain estimate.